Physicists Take Big Step in Nanolaser Design

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Nanolasers have recently emerged as a new class of light sources that have a size of only a few millionths of a meter and unique properties remarkably different from those of macroscopic lasers. However, it is almost impossible to determine at what current the output radiation of the nanolaser becomes coherent, while for practical applications, it is important to distinguish between the two regimes of the nanolaser: the true lasing action with a coherent output at high currents and the LED-like regime with incoherent output at low currents. Researchers from the Moscow Institute of Physics and Technology developed a method that allows to find under what circumstances nanolasers qualify as true lasers. The research was published in Optics Express.

Lasers are widely used in household appliances, medicine, industry, telecommunications, and more. Several years ago, lasers of a new kind were created, called nanolasers. Their design is similar to that of the conventional semiconductor lasers based on heterostructures, which have been known for several decades. The difference is that the cavities of nanolasers are exceedingly small, on the order of the wavelength of the light emitted by these light sources. Since they mostly generate visible and infrared light, the size is on the order of one millionth of a meter. 

In the near future, nanolasers will be incorporated into integrated optical circuits, where they are required for the new generation of high-speed interconnects based on photonic waveguides, which would boost the performance of CPUs and GPUs by several orders of magnitude. In a similar way, the advent of fiber optic internet has enhanced connection speeds, while also boosting energy efficiency. 

And this is by far not the only possible application of nanolasers. Researchers are already developing chemical and biological sensors, mere millionths of a meter large, and mechanical stress sensors as tiny as several billionths of a meter. Nanolasers are also expected to be used for controlling neuron activity in living organisms, including humans. 

For a radiation source to qualify as a laser, it needs to fulfill a number of requirements, the main one being that it has to emit coherent radiation. One of the distinctive properties of a laser, which is closely associated with coherence, is the presence of a so-called lasing threshold. At pump currents below this threshold value, the output radiation is mostly spontaneous and it is no different in its properties from the output of conventional light emitting diodes (LEDs). But once the threshold current is reached, the radiation becomes coherent. At this point the emission spectrum of a conventional macroscopic laser narrows down and its output power spikes. The latter property provides for an easy way to determine the lasing threshold — namely, by investigating how output power varies with pump current (figure 1A).
   

Figure 1. Dependence of the output power on pump current for a conventional macroscopic laser (A), and for a typical nanoscale laser (B) at a given temperature. Image courtesy of the researchers 

Many nanolasers behave the way their conventional macroscopic counterparts do, that is, they exhibit a threshold current. However, for some devices, a lasing threshold cannot be pinpointed by analyzing the output power versus pump current curve, since it has no special features and is just a straight line on the log-log scale (red line in figure 1B). Such nanolasers are known as “thresholdless.” This begs the question: At what current does their radiation become coherent, or laserlike? 

The obvious way to answer this is by measuring the coherence. However, unlike the emission spectrum and output power, coherence is very hard to measure in the case of nanolasers, since this requires equipment capable of registering intensity fluctuations at trillionths of a second, which is the timescale on which the internal processes in a nanolaser occur.

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